Definitions can be used as expressions

[shawwn]

In Arc, afn is used to create a recursive function:

arc> ((afn (n)
        (if (< n 2) n
            (+ (self (- n 1))
               (self (- n 2)))))
      8)
21

This PR lowers definitions as expressions, achieving the same thing in a natural way:

diff --git a/test.l b/test.l
index 80b089a..e71f09c 100755
--- a/test.l
+++ b/test.l
@@ -581,7 +581,13 @@ c"
   (test= 42 (f))
   ((fn ()
      (define f () 38)
-     (test= 38 (f))))
+     (test= 38 (f))
+     (test= 21 ((define f (n)
+                  (if (< n 2) n
+                    (+ (f (- n 1))
+                       (f (- n 2)))))
+                8))
+     (test= 21 (f 8))))
   (test= 42 (f)))